Process for the manufacture of 1-chloro-1,1-difluoroethane

ABSTRACT

1-Chloro-1,1-difluoroethane is prepared by reacting hydrogen fluoride with vinylidene chloride, in a liquid medium containing preferably at least 40 mol % 1,1-dichloro-1-fluoroethane.

This is a continuation of application Ser. No. 06/672,177 filed on11/15/84 which in turn in a continuation of application Ser. No.06/572,132 filed on 01/19/84 which in turn is a continuation ofapplication Ser. No. 06/407,078 filed on 08/11/82 which in turn is acontinuation of application Ser. No. 06/268,702 filed on 06/01/81 whichin turn is a continuation of application Ser. No. 05/824,320 filed on08/12/77 all now abandoned.

The present invention relates to a process for the manufacture of1-chloro-1,1-difluoroethane from vinylidene chloride by reaction withhydrogen fluoride, if appropriate in the presence of a catalyst.

1-Chloro-1,1-difluoroethane is an intermediate product of the synthesisof vinylidene fluoride, which is itself used for the manufacture ofpolymers.

The known processes of fluorination of vinylidene chloride by means ofhydrogen fluoride have hitherto not made it possible to obtain highyields of 1-chloro-1,1-difluoroethane. In fact, under mild conditions,only the addition product, namely 1,1-dichloro-1-fluoroethane can beobtained. Furthermore, if very active catalysts, high molecular ratiosof hydrogen fluoride:vinylidene chloride, high temperatures, or severalof these conditions simultaneously are used, undesired products such as1,1,1-trifluoroethane, as well as a large amount of oligomers, areobtained.

To favour the formation of 1-chloro-1,1-difluoroethane, it has beenproposed, in Japanese Patent Application No. 39,086/72 filed on Aug. 31,1964 in the name of Kureha Chemical Ind. Co., to carry out the reactionin the presence of tin tetrachloride, with a molar ratio of hydrogenfluoride:vinylidene chloride greater than 4, and at a temperature above60° C. However, this process does not make it possible completely toavoid the formation of undesirable by-products. Thus, in this process, asubstantial formation of 1,1,1-trifluoroethane is observed, in spite ofa very low degree of fixing of the hydrogen fluoride.

There has now been found, in accordance with the present invention aprocess for the manufacture of 1-chloro-1,1-difluoroethane byhydrofluorination of vinylidene chloride which does not exhibit thedisadvantages, mentioned above, of the known processes.

Accordingly, the present invention relates to a process for themanufacture of 1-chloro-1,1-difluoroethane by reaction of hydrogenfluoride with vinylidene chloride, in which the reaction is carried outin a liquid medium containing 1,1-dichloro-1-fluoroethane.

The number of mols of 1,1-dichloro-1-fluoroethane present in the liquidmedium is at least 40 mol. %, preferably greater than 40% relative tothe total number of mols of organic compounds participating in thereaction and present in the liquid medium. The best results are obtainedwhen this amount is greater at least 60 mol. %, or than 60%. However, abeneficial effect is observed even from 20% onwards. By organiccompounds participating in the reaction there are understood vinylidenechloride, 1,1-dichloro-1-fluoroethane, 1-chloro-1,1-difluoroethane andall the by-products of the reaction.

In addition to 1,1-dichloro-1-fluoroethane, the liquid medium inparticular contains vinylidene chloride and the products of thereaction; it can also possibly contain one or more liquid solvents whichare inert under the reaction conditions and have a boiling point abovethat of 1-chloro-1,1-difluoroethane under the pressure conditions of thereaction. The content of vinylidene chloride in the liquid mediumpreferably does not exceed 50 mol % relative to the total number of molsof organic compounds participating in the reaction and present in themedium. The best results are obtained when it does not exceed 30%. Infact, if the vinylidene chloride content is higher, the formation oflarge amounts of oligomeric by-products is observed.

The amount of 1-chloro-1,1-difluoroethane in the liquid mediumpreferably does not exceed 50 mol % relative to the total number of molsof organic compounds participating in the reaction and present in theliquid medium. The best results are obtained if it does not exceed 30%.In fact, if the content of 1-chloro-1,1-difluoroethane is higher, theformation of undesirable highly fluorinated products is observed.

The liquid medium furthermore contains hydrogen fluoride as well as,optionally, various other additives and, in particular, catalysts.

As catalysts it is possible to use any hydrofluorination catalyst whichfavours the replacement of a chlorine atom by a fluorine atom. Catalystswhich simultaneously favour the addition of the hydrogen fluoride to adouble bond and the replacement of a chlorine atom by a fluorine atomare very suitable. Amongst the catalysts which can be used there may bementioned the compounds of the elements chosen from amongst the elementsof groups IIIa and b, IVa and b, Va and b, VIa and b, VIIb and VIII ofthe periodic table of the elements and more particularly from amongstlanthanum, boron, aluminium, gallium, titanium, tin, vanadium, bismuth,arsenic, antimony, chromium, sulphur, manganese, iron, cobalt andnickel. Titanium, tin, vanadium, bismuth, arsenic, antimony, chromiumcompounds are generally used. The tin compounds are particularlysuitable. The compounds used are preferably the halides, such as thechlorides and the fluorides, as well as the oxides and the oxyhalidesand preferably the oxychlorides and oxyfluorides. Tin tetrachloride hasproved particularly valuable.

The catalyst or catalysts are usually employed at the rate of 0.001 to 5and preferably of 0.01 to 2 mols per kg of liquid medium.

In order to maintain a sufficient content of 1,1-dichloro-1-fluoroethanein the liquid medium, a simple means consists of continuouslywithdrawing the 1-chloro-1,1-difluoroethane from the reaction medium sothat the latter does not accumulate in the reactor. The1-chloro-1,1-difluoroethane can be removed from the liquid medium invarious ways. Thus, it is possible continuously to take a part of theliquid medium and to subject it to a separation in order separately tocollect 1-chloro-1,1-difluoroethane and recycle, to the reactor, aliquid enriched in 1,1-dichloro-1-fluoroethane. It is also possible,according to a preferred embodiment of the invention, to use atemperature and a pressure such that the 1-chloro-1,1-difluoroethanecontinuously leaves the liquid medium in the form of a gas. In thislatter case, it is also possible simultaneously to withdraw a part ofthe liquid medium and subject it to a separation as described above.

The reaction temperature is generally chosen to be between 30° and 180°C. and preferably between 40° and 160° C. The reaction pressure ischosen so as to maintain the reaction medium in the liquid form. It ismost commonly between 2 and 80 kg/cm² and varies according to thetemperature of the reaction medium. If it is desired continuously towithdraw a part of the liquid phase in order to remove the1-chloro-1,1-difluoroethane therefrom, it is preferable to use highpressures, for example between 5 and 80 kg/cm². On the other hand if,according to the preferred embodiment of the invention, it is desired towithdraw the 1-chloro-1,1-difluoroethane in the gaseous form from thereaction medium, lower pressures, for example between 2 and 50 kg/cm²,are used. In this latter case, pressures of between 3 and 30 kg/cm² haveproved advantageous.

According to the preferred embodiment of the invention, the reactiontemperature and pressure are chosen so as, on the one hand, to ensurethat the reaction medium is kept in the liquid phase and, on the otherhand, to allow the 1-chloro-1,1-difluoroethane to leave the reactionmedium in the gaseous form, whilst maintaining the vinylidene chloride,1,1-dichloro-1-fluoroethane and hydrogen fluoride in the liquid form.

The molar ratio of hydrogen fluoride:vinylidene chloride is generallychosen to be above 1.5. Most frequently, molar ratios of between 1.5 and4, and preferably between 1.75 and 3.7, are used. The rates ofintroduction of the reactants are so regulated as to maintain thedesired proportions for the compounds present in the reaction medium andespecially so as to avoid the accumulation of vinylidene chloride in theliquid medium.

The reaction can be carried out in various types of reactors which arein themselves known. Thus it is possible to introduce the hydrogenfluoride and vinylidene chloride in parallel at the bottom of a verticalreactor containing 1,1-dichloro-1-fluoroethane. It is also possible tocause hydrogen fluoride and vinylidene chloride to flow through thereactor in counter-current.

The reactor is usually made from materials which are resistant topressure and to hydrogen fluoride. In general, reactors made of steel,stainless steel, nickel, copper or an alloy containing chromium,chromium and nickel, nickel and copper or even molybdenum are used.Alloys such as MONEL, INCONEL and HASTELLOY are very suitable. It isalso possible to use reactors equipped with a lining of an inert metalor alloy, or coated with a layer of a resin which is inert under thereaction conditions, such as, for example, phenoplasts or fluorinatedresins.

The reactors are advantageously equipped with devices which are inthemselves known and make it possible to improve the contact between thehydrogen fluoride and the reactant. It is thus possible to providestirrers in the reactor or to provide means of introducing the reactantswhich allow efficient dispersion of the latter in the reaction medium.

One of the reactants, or both reactants and/or the catalyst canoptionally be introduced at several spaced-out points of the reactor.Thus it is possible and valuable to equip the reactor with several meansof introducing the vinylidene chloride.

The reaction can be carried out in a single reactor or in severalreactors arranged in series. In this case, it is possible to providevarious means of introducing the reactants and the catalyst. The tworeactants can be introduced into one and the same reactor, the catalystbeing divided amongst all the reactors. It is also possible to introduceat least one of the two reactants into each of the reactors. It is alsopossible to use two reactors respectively fed with each of thereactants, the two mixtures formed in each of the reactors flowing incounter-current.

When the 1-chloro-1,1-difluoroethane leaves the reaction medium in thegaseous form, a gas phase which furthermore contains hydrogen chloride,volatile by-products such as 1,1,1-trifluoroethane as well as a littlehydrogen fluoride, 1,1-dichloro-1-fluoroethane, vinylidene chloride andpossibly 1,1,1-trichloroethane as a by-product of the reaction iscollected. This mixture can be subjected to one or more separations soas to collect the 1-chloro-1,1-difluoroethane which is the product ofthe reaction, whilst the hydrogen fluoride, 1,1-dichloro-1-fluoroethane,vinylidene chloride and any 1,1,1-trichloroethane can be recycled to thereactor.

It is thus possible to subject the gas mixture coming from the reactorto a fractional distillation so as to separate the hydrogen chloridefrom a mixture containing the hydrogen fluoride and the organiccompounds. This mixture can then be decanted to separate the hydrogenfluoride from the organic phase, the latter being subjected to severalsuccessive distillations which make it possible to collect the1-chloro-1,1-difluoroethane, which is the reaction product, and the1,1-dichloro-1-fluoroethane as well as the vinylidene chloride, whichare recycled to the reactor at the same time as the unconverted hydrogenfluoride.

The attached FIGS. 1 and 2 schematically represent valuable embodimentsof the process according to the present invention.

According to the diagram in FIG. 1, hydrogen fluoride is introduced intoa reactor 1, containing 1,1-dichloro-1-fluoroethane in the liquid phase,through line 2 and vinylidene chloride through line 3. The gases whichhave left the reactor through line 4 pass into a condenser 7 from whicha liquid phase is withdrawn, which is recycled through line 6 to thereactor 1, and a gaseous phase is withdrawn through line 5. The gaseousphase which is withdrawn at 5 contains principally hydrogen chloride,1-chloro-1,1-difluoroethane, 1,1,1-trifluoroethane as a by-product andpossibly also a little hydrogen fluoride, 1,1-dichloro-1-fluoroethaneand vinylidene chloride.

In the process represented in FIG. 2, hydrogen fluoride is introducedinto a reactor 1, containing 1,1-dichloro-1-fluoroethane in the liquidphase, through line 2 and vinylidene chloride through line 3. The gaseswhich have left the reactor through line 4 pass into a condenser 7, fromwhich a liquid phase is withdrawn, which is recycled through line 6 tothe reactor 1, and a gas phase containing hydrogen chloride,1,1,1-trifluoroethane, 1-chloro-1,1-difluoroethane and a little hydrogenfluoride, 1,1-dichloro-1-fluoroethane and vinylidene chloride is alsowithdrawn. This gaseous phase is withdrawn through line 5 and passedinto a separation zone 9 from which hydrogen chloride,1,1,1-trifluoroethane and 1-chloro-1,1-difluoroethane are withdrawn at11 and a mixture containing hydrogen fluoride, vinylidene chloride and1,1-dichloro-1-fluoroethane is withdrawn at 10. The separation zone 9can in particular be a distillation zone. It can also be replaced by anassembly of separation apparatuses comprising, for example, distillationcolumns, scrubbers and decanters so arranged as to separate the gaseousmixture into all its constituents.

The process according to the present invention has proved particularlyvaluable. In fact, it makes it possible substantially to reduce theformation of oligomers. Furthermore, very little 1,1,1-trifluoroethaneis formed, in spite of a high degree of utilisation of the hydrogenfluoride. The process of the invention thus makes it possible to achievevery high yields of 1-chloro-1,1-difluoroethane. The latter isadvantageously used in the manufacture, by dehydrochlorination, ofvinylidene fluoride, which is a monomer used for the preparation ofpolymers having high chemical resistance and high heat resistance.

Example 2 which follows, and which is in no way limiting in character,shows the remarkable results obtained in accordance with an embodimentof the invention. Example 1R is given by way of comparison.

EXAMPLE 1 (R)

By way of comparison, an experiment was carried out discontinuously, ina liquid medium which at the start of the reaction only containedvinylidene chloride, hydrogen fluoride and catalyst.

The experiment was carried out at 60° C. in a 3.5 l stainless steelreactor initially containing 5.16 mols of vinylidene chloride, 16.05mols of hydrogen fluoride and 0.25 mol of tin tetrachloride. The reactoris equipped with a stirrer rotating at 290 revolutions per minute andsurmounted by a condenser, the outlet temperature of which is about -20°C. The pressure in the reactor is kept at 5.4 atmospheres absolutewhich, in view of the formation of hydrogen chloride during thereaction, necessitates releasing a part of the contents of the reactor.The organic gases released, as well as the organic compounds present inthe reactor at the end of the experiment, are collected and analysed.The analysis of all the products obtained after 1 hour 30 minutes ofreaction is given in Table I below:

                  TABLE I    ______________________________________                     1 R                       total                       organic                       compounds    Experiment         (mols)    mol %    ______________________________________    1,1,1-trifluoroethane                       0.111     2.1    1-chloro-1,1-difluoroethane                       1.377     26.7    1,1-dichloro-1-fluoroethane                       3.339     64.8    vinylidene chloride                       0.218     4.1    1,1,1-trichloroethane                       0.041     0.8    oligomers (equivalents of                       0.070     1.4    ethane derivatives)    ______________________________________

The degree of conversion of the vinylidene chloride is 95.8%.

The molar ratio of 1,1,1-trifluoroethane:1-chloro-1,1-difluoroethane is0.080.

The degree of conversion of the vinylidene chloride to oligomers is1.4%.

The oligomers were determined by vapour phase chromatography andidentified by mass spectrometry. They consist of molecules of theformula

    C.sub.4 H.sub.5 Cl.sub.x F.sub.5-x where 1<x<4

    C.sub.6 H.sub.7 Cl.sub.y F.sub.7-y where 0<y<4

    C.sub.8 H.sub.9 Cl.sub.z F.sub.9-z where 2<z<3

EXAMPLE 2

The experiments below were carried out in accordance with the invention,in a device similar to that shown in FIG. 1.

2 l of a liquid bottom layer are placed in the reactor, having a volumeof 3.5 l , which is made of stainless steel. Two tubes dipping to thebottom of the reactor make it possible to introduce hydrogen fluorideand vinylidene chloride. The reactor is equipped with a stirrer turningat 290 revolutions per minute and is surmounted by a condenser.

The working conditions and the results obtained are shown in Table IIbelow.

                  TABLE II    ______________________________________    Experiment No.         2       3      4    ______________________________________    Experimental conditions    Reaction temperature                   °C.  60      60   60    Temperature at condenser                   °C.  40-50   60   60    outlet    Pressure       atmospheres 5.4     7.0  7.0                   absolute    Catalyst content                   mol/kg of liq-                               0.5     0.5  0.5                   uid medium    Volume of the liquid                   1           2       2    2    medium    Composition of the liquid    medium    1,1-dichloro-1-fluorethane                   mol % of    84.2    76.01                                            80.3    1-chloro-1,1-difluoroethane                   organic     10.1    12.6 14.1    vinylidene chloride                   compounds   3.2     8.13 2.0    Feed of reactants    Vinylidene chloride                   mol/hour    6.37    6.51 6.16    Hydrogen fluoride                   mol/hour    18.83   18.25                                            17.73    Hydrogen fluoride:                   mol/hour    2.9     2.8  2.9    vinylidene chloride    Products formed    1,1,1-trifluoroethane                   mol/hour    0.060   0.100                                            0.157    1-chloro-1,1-difluoroethane                               5.279   5.224                                            5.252    1,1-dichloro-1-fluoroethane                               0.983   1.154                                            0.732    vinylidene chloride        0.022   0.023                                            0.004    1,1,1-trichloroethane      0.004   0.005                                            0.007    oligomers (equivalent of   0.027   0.007                                            0.011    ethane derivatives)    Degree of conversion of the                   mol %       99.7    99.6 99.9    vinylidene chloride    Degree of conversion of the                               0.4     0.1  0.2    vinylidene chloride to    oligomers    Degree of conversion of the                               0.9     1.5  2.5    vinylidene chloride to    1,1,1-trifluoroethane    Selectivity in respect of  83.1    80.5 85.3    1-chloro-1,1-difluoroethane    Selectivity in respect of  98.6    98.3 97.2    (1-chloro-1,1-    difluoroethane + 1,1-    dichloro-1-fluoroethane    ______________________________________

Furthermore, comparison of Tables I and II shows that in experiments 2,3 and 4, molar ratios of 1,1,1-trifluoroethane:1-chloro-1,1-difluoroethane much lower (respectively 0.011, 0.019 and0.029) than in the comparison experiment 1R (0.080) are obtained. Theformation of 1,1,1-trifluoroethane relative to the desired product isthus much less in the process of the invention than in the comparisonprocess.

We claim:
 1. Continuous process for the manufacture of1-chloro-1,1-difluoroethane by reacting hydrogen fluoride withvinylidene chloride, wherein the reaction is carried out in a liquidmedium in the presence of a hydrofluorination catalyst which is a tincompound, comprising maintaining in the liquid medium a content of atleast 40 mol % of 1,1-dichloro-1-fluoroethane, no more than 30 mol % ofvinylidene chloride and no more than 30 mol % of1-chloro-1,1-difluoroethane, all relative to the total number of molesof organic compounds participating in the reaction and present in theliquid medium, while feeding into the liquid medium hydrogen fluorideand vinylidene chloride in a hydrogen fluoride/vinylidene chloride molarratio of between 1.5 and 3.7 by continuously withdrawing1-chloro-1,1-difluoroethane from the liquid medium.
 2. Process accordingto claim 1, wherein the liquid medium contains at least 60 mol % of1,1-dichloro-1-fluoroethane relative to the total number of moles oforganic compounds participating in the reaction and present in theliquid medium.
 3. Process according to claim 1, wherein the catalyst istin tetrachloride.
 4. Process according to claim 1, wherein from 0.001to 5 moles of catalyst are used per kg of liquid medium.
 5. Processaccording to claim 2, wherein from 0.01 to 2 moles of catalyst are usedper kg of liquid medium.
 6. Process according to claim 1, wherein thereaction as carried out at a temperature of between 30° and 180° C. 7.Process according to claim 6, wherein the reaction is carried out at atemperature of between 40° and 160° C.
 8. Process according to claim 1,wherein the reaction is carried out at a pressure of between 2 and 80kg/cm².
 9. Process according to claim 8, wherein the reaction is carriedout at a pressure of between 3 and 30 kg/cm².
 10. Process according toclaim 1, wherein 1-chloro-1,1-difluoroethane is withdrawn in a gaseousphase.
 11. Process according to claim 1, wherein at least a part of the1-chloro-1,1-difluoroethane is withdrawn in a liquid phase.
 12. Processaccording to claim 1, wherein the molar ratio of hydrogen fluoride tovinylidene chloride is between 1.75 and 3.7.
 13. Process according toclaim 1, wherein the liquid medium has a content of from 76.01 to 84.2mol % of 1,1-dichloro-1-fluoroethane relative to the total number ofmoles of organic compounds participating in the reaction and present inthe liquid medium and the molar ratio of hydrogen fluoride to vinylidenechloride is 2.8 to 2.9.
 14. Process according to claim 1, wherein thegaseous phase containing 1-chloro-1,1-difluoroethane is separated intoits components, 1,1-dichloro-1-fluoroethane is collected, and thecollected 1,1-dichloro-1-fluoroethane is recycled to the liquid reactionmedium.
 15. Process according to claim 11, wherein the phase containing1-chloro-1,1-difluoroethane is separated into its components,1,1-dichloro-1-fluoroethane is collected, and the collected1,1-dichloro-1-fluoroethane is recycled to the liquid reaction medium.16. Process according to claim 1, wherein the1-chloro-1,1-difluoroethane is continuously withdrawn as both liquid andgaseous phases from the liquid medium.
 17. Process according to claim16, wherein both said liquid and gaseous phases are subjected to aseparation to collect 1,1-dichloro-1-fluoroethane and the collected1,1-dichloro-1-fluoroethane is recycled to the liquid reaction medium.18. Continuous process for the manufacture of1-chloro-1,1-difluoroethane by reacting hydrogen fluoride withvinylidene chloride, wherein the reaction is carried out in a liquidmedium in the presence of a hydrofluorination catalyst which is a tincompound, said reaction forming1-chloro-1,1-difluoroethane,1,1-dichloro-1-fluoroethane,1,1,1-trifluoroethaneand oligomers, comprising maintaining in the liquid medium a contentgreater than 40 mol % of 1,1-dichloro-1-fluoroethane, no more than 50mol % of vinylidene chloride and no more than 50 mol % of1-chloro-1,1-difluoroethane, all relative to the total number of molesof organic compounds participating in the reaction and present in theliquid medium, while feeding into the liquid medium hydrogen fluorideand vinylidene chloride in a hydrogen fluoride/vinylidene chloride molarratio between 1.5 and 3.9 by continuously withdrawing1-chloro-1,1-difluoroethane from the liquid medium, said withdrawingprocess step defining very high yields of 1-chloro-1,1-difluoroethanewhile forming very little 1,1,1-trifluoroethane.
 19. Process accordingto claim 18, said process further defining a molar ratio of1,1,1-trifluoroethane: 1-chloro-1,1-difluoroethane is less than 0.080.20. Process according to claim 15, wherein said molar ratio of1,1,1-trifluoroethane: 1-chloro-1,1-difluoroethane is less than 0.029.21. Continuous process for the manufacture of1-chloro-1,1-difluoroethane by reacting hydrogen fluoride withvinylidene chloride, wherein the reaction is carried out in a liquidmedium in the presence of a hydrofluorination catalyst which is a tincompound, said reaction forming1-chloro-1,1-difluoroethane,1,1-dichloro-1-fluoroethane,1,1,1-trifluoroethaneand oligomers, comprising maintaining in the liquid medium a contentgreater than 40 mol % of 1,1-dichloro-1-fluoroethane, no more than 50mol % of vinylidene chloride and no more than 50 mol % of1-chloro-1,1-difluoroethane, all relative to the total number of molesof organic compounds participating in the reaction and present in theliquid medium, while feeding into the liquid medium hydrogen fluorideand vinylidene chloride in a hydrogen fluoride/vinylidene chloride molarratio between 1.5 and 3.7, by continuously withdrawing1-chloro-1,1-difluoroethane from the liquid medium, said withdrawingprocess step defining very high yields of 1-chloro-1,1-difluoroethanewhile substantially reducing the formation of oligomers.
 22. The processof claim 21, wherein the degree of conversion of vinylidene chloride tooligomers is 1.4 mol %.
 23. Continuous process for the manufacture of1-chloro-1,1-difluoroethane by reacting hydrogen fluoride withvinylidene chloride, wherein the reaction is carried out in a liquidmedium in the presence of a hydrofluorination catalyst which is a tincompound, said reaction forming1-chloro-1,1-difluoroethane,1,1-dichloro-1-fluoroethane,1,1,1-trifluoroethaneand oligomers, comprising maintaining in the liquid medium a contentgreater than 40 mol % of 1,1-dichloro-1-fluoroethane, no more than 50mol % of vinylidene chloride and no more than 50 mol % of1-chloro-1,1-difluoroethane, all relative to the total number of molesof organic compounds participating in the reaction and present in theliquid medium, while feeding into the liquid medium hydrogen fluorideand vinylidene chloride in a hydrogen fluoride/vinylidene chloride molarratio between 1.5 and 3.7, by continuously withdrawing1-chloro-1,1-difluoroethane from the liquid medium, said withdrawingprocess step defining very high yields of 1-chloro-1,1-difluoroethane.24. The process of claim 19, wherein said withdrawing process having aproduct yield of at least 80.5 mol % of 1-chloro-1,1-difluoroethane. 25.The process of claim 19, wherein said withdrawing process stepsubstantially reduces the formation of 1,1,1-trifluoroethane andoligomers.
 26. The process of claim 21, wherein the degree of vinylidenechloride conversion to the combination of 1,1,1-trifluoroethane andoligomers is equal to or less than 2.7 mol %.
 27. The process of claim20, wherein said liquid medium has a 60 mol % of1,1-dichloro-1-fluoroethane, no more 30 mol % of vinylidene chloride andno more than 30 mol % of 1-chloro-1,1-difluoroethane.